This application relates to signal oscillators.
Signal oscillators may be constructed by using both electronic and optical components to form opto-electronic oscillators (OEOs). See, e.g., U.S. Pat. Nos. 5,723,856 and 5,777,778, which are incorporated herein by reference as part of the specification of this application. Such an OEO may include an electrically controllable optical modulator and at least one active opto-electronic feedback loop that includes an optical part and an electrical part interconnected by a photodetector. The opto-electronic feedback loop receives the modulated optical output from the modulator and converted it into an electrical signal to control the modulator. The loop produces a desired delay and feeds the electrical signal in phase to the modulator to generate and sustain both optical modulation and electrical oscillations outside the optical in, e.g., the microwave or radio frequency spectral range when the total loop gain of the active opto-electronic loop and any other additional feedback loops exceeds the total loss. The generated oscillating signals may be tunable in frequency and can have narrow spectral linewidths and low phase noise in comparison with the signals produced by other RF and microwaves oscillators. Notably, the OEOs are optical and electronic hybrid devices.
An OEO may be configured as a coupled opto-electronic oscillator (COEO) which directly couples a laser oscillation in an optical feedback loop (e.g., a laser cavity) to an electrical oscillation in an opto-electronic feedback loop. See, e.g., U.S. Pat. Nos. 5,929,430 and 6,567,436, which are incorporated herein by reference as part of the specification of this application. An optical resonator may be used in an optical section in which the optical feedback loop and the opto-electronic feedback loop overlap. The optical resonator may be an optical whispering-gallery-mode (“WGM”) resonator which supports a special set of resonator modes known as whispering gallery (“WG”) modes. These WG modes represent optical fields confined in an interior region close to the surface of the resonator due to the total internal reflection at the boundary. Optical WGM resonators with high quality factors have been demonstrated at Q values greater than 109. Such hi-Q WGM resonators may be used to produce oscillation signals with high spectral purity and low noise.